I think we should have a section in css3-break (perhaps 5.4?) that defines
what happens when content in a particular fragment is displayed outside
the fragmentainer boundary. Two instances of this are relatively
positioned elements and line boxes at the top of the fragmentainer with a
line-height insufficient to enclose the content. The test file in a recent
css3-regions thread [1] shows non-interoperable behavior with multicol
elements across all browsers.
I think some of the differences are just bugs. But what happens with
relatively-positioned elements appears to be a result of two solutions
arrived at separately because there is no spec that says what should
happen. Since this is shared behavior between columns and regions, I think
it belongs in css3-break.
The question on relative positioning is whether the offsets apply relative
to the fragmentainer or the fragmented flow. In the former solution, a
line box shifted down at the bottom of a fragmentainer would appear below
the fragmentainer box. In the latter solution the line box would appear at
the top of the next fragmentainer in the fragmentation context. I think
that the former solution is preferable.
Here's my initial stab at some text to add for relative positioning:
---
Once the fragment contents for a particular fragmentainer are
determined,
any offsets from relative positioning are applied relative to the
position
of the content in the fragmentainer, not the position of the content in
the fragmented flow.
---
Thanks,
Alan
[1] http://lists.w3.org/Archives/Public/www-style/2012Sep/0268.html